1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus equipped with the same, such as a video camera and a digital camera.
2. Description of the Related Art
Electronic image pickup apparatuses such as digital cameras and video cameras have become the mainstream in recent years. The electronic image pickup apparatus picks up an image of an object using an image pickup element such as a CCD sensor or a CMOS sensor instead of a film.
Various types of electronic image pickup apparatuses including, for example, high-performance professional-use digital cameras, compact popular-priced digital cameras, and surveillance cameras are widely used.
Since there are various shooting situations, zoom lenses that provide changes in the angle of view find favor commonly for use in such electronic image pickup apparatuses.
Furthermore, there is a need for slimming and size reduction of electronic image pickup apparatuses and for improvement in the image quality.
However, reducing the pixel pitch of the image pickup element will lead to a low sensitivity due to a decrease in the signal-to-noise ratio, which tends to invite a deterioration in the image quality caused by motion blur and/or hand shake.
Motion blur cannot be eliminated or reduced by hand shake reduction methods using a mechanical correction mechanism that shifts a part of the optical system or the image pickup element, because the exposure time cannot be shortened. A reduction or elimination of motion blur requires a complicated control system.
One possible solution is to detect a motion of an object by a sensor or the like and shifts the optical system and/or the image pickup element relative to the body of the image pickup apparatus. Another possible solution is performing correction of an image by electrical computation after shooting an object.
A known solution is to increase the sensitivity of the image pickup element to shorten the exposure time. This solution is effective in reducing both motion blur and hand shake. However, shortening of the exposure time leads to a decrease in the quantity of light received by each light receiving element arranged on the image pickup surface of the image pickup element. Consequently, the signal-to-noise ratio becomes lower, and the image quality tends to be deteriorated by noises.
To compensate insufficient sensitivity, a zoom lens having a fast (i.e. small) F-number is required.
Zoom lenses used in compact digital cameras typically have a zoom ratio of approximately 3 (three) and a wide angle of view at the wide angle end of approximately 63°.
To enable satisfactory shooting in various situations, zoom lenses having a high zoom ratio and a wide angle of view at the wide angle end are demanded.
It is known that to achieve a fast F-number, a type of zoom lenses in which the first lens unit (i.e. the frontmost lens unit) has a positive refracting power are advantageous. However, in this type of zoom lenses, the diameter of the first lens unit tends to be large, and the total number of lenses tends to be large. For this reason, this type of zoom lens is not suitable as a zoom lens that is collapsible into a thin (or slim) form for housing in the image pickup apparatus when the apparatus is not in use.
On the other hand, a type of zoom lenses in which the first lens unit has a negative refracting power (which are sometimes called negative-lead type zoom lenses) are known as zoom lenses that allow housing in a compact form. This type of zoom lenses are used many in slim cameras, because they can be made thinner, when collapsed, than the zoom lenses in which the first lens unit has a positive refracting power.
As such zoom lenses, zoom lenses in which the lens unit closest to the object side has a negative refracting power, and the distances between lens units change during zooming from the wide angle end to the telephoto end are disclosed in Japanese Patent Application Laid-Open Nos. 2004-318099, Hei 1-40913, and 2001-208969.